1. Field of the Invention
The invention relates generally to electrical apparatus, and more particularly, to enclosed circuit interrupters with fuse protection.
2. Description of the Prior Art
In supplying the nation's energy needs, two primary objectives of the electric utility industry are safety and reliability. Since the late 1920's, the AC secondary network system has been used in downtown business districts and commercial areas to provide a high degree of service continuity. In the AC secondary network system, the secondary mains surrounding the area being served, such as a city block, are connected together to form a secondary network grid or mesh at low voltage from which the customer loads are viewed. The secondary network is supplied over a plurality of high-voltage transmission lines or feeders through network transformers. The transformers reduce the high-voltage necessary for transmission of electric energy to lower voltage suitable for distribution to customers. In the AC seconary network system, the failure of any one feeder will not cause interruption to service because the load will be supplied over the remaining feeders. When a failure, or fault, occurs in a high-voltage feeder or in one of its associated network transformers, the station end of the feeder, that is, the end closest to the generating station, is disconnected from the system by opening of the feeder circuit breaker. In addition, it is necessary that all network transformers on the failed feeder be disconnected from the network by some form of protective device to prevent power from the network from being fed back through the network grid and transformer to the fault. The automatic network protector was developed for this purpose. The network protector consists of a specially designed air circuit breaker with a closing and opening mechanism controlled by a network master relay and a network phasing relay. When the network protector is closed, the master relay functions to trip it on a reversal of power flow. The master relay and the phase relay act together to close the protector when, and only when, the correct voltage conditions exist across it.
Network protectors are often located outdoors or underground in vaults, thus they must be protected by a sealed enclosure. When approached for service, test, or maintenance, network protectors must be electrically and physically disconnected from the system on both the transformer and the load side. Historically, this consideration dictated the use of a rollout type breaker design which could be disconnected and rolled out of its enclosure for inspection and maintenance. Network fuses which were located within the enclosure on the load or network side of the rollout units, as well as the disconnect links located on the line or transformer side, had to be unbolted after failure using insulated tools and removed from the network protector by hand. This operation presented a potential hazard to maintenance personnel since an accidentally dropped piece of hardware, a tool, a fuse, or disconnect link might fall into the protector, despite a protective barrier system, and come in contact with components at high potential. The resulting arc-over often resulted in injury to maintenance personnel and damage to the equipment.
In order to overcome the limitations of prior art network protectors, it would be desirable to provide a lower cost fused network protector which would allow maintenance with an increased degree of safety and convenience.